1. Field of the invention
This invention relates to a biaxially oriented saturated polyester film with heat seal, a method of making the same, a laminate using the biaxially oriented saturated polyester film and a method of making the same.
2. Description of the related art
A biaxially oriented saturated polyester film such as saturated polyethylene terephthalate (hereinafter referred to as “PET”) has various superior characteristics and is accordingly used in various fields. For example, the biaxially oriented saturated polyester film is used for wrapping food or the like because of its high transparency, small permeability and high safety and sanitation. Furthermore, the biaxially oriented saturated polyester film is used as a material for capacitors because of its toughness and superior electric characteristics, as well known in the art.
However, molecules of the biaxially oriented saturated polyester film are oriented and crystallized during orientation though the biaxially oriented saturated polyester film has superior characteristics as described above. As a result, a sheet of biaxially oriented saturated polyester film cannot be heat-sealed directly with another sheet of biaxially oriented saturated polyester film, different type of film or metal foil at or below a melting point of the biaxially oriented saturated polyester film (at or below 256° C. in PET, for example). Thus, PET has no heat sealability as well known in the art. In view of this problem, the prior art has proposed provision of a heat-seal layer made by coating a copolymer of polyester resin on one side of a PET film and having a thickness of about 1 μm so that the PET film may have a heat sealability in the use of a food wrapping film. This is described on page 40, “Convertech,” published in September 2002 by Converting Technical Institute, Tokyo.
“Convertech” published in May 2003 by Converting Technical Institute further proposes alternative method (page 68, TABLE 3). In the proposed method, a polybutadiene or urethane anchor coat is applied to the PET film and thereafter, an aluminum foil is bonded to the PET film using LDPE (low density polyethylene) with heat-sealability. Furthermore, JP-B-3-19052 discloses further another method in which a surface of the PET film is plasma-treated at a low temperature and a heat-sealable bonding agent layer is then provided so that the PET films are heated or the PET film is heated together with a metal, aluminum foil or polycarbonate or the like so as to be heated and pressed thereby to be laminated.
However, the above-described prior art methods each necessitate provision of the bonding layer comprising different types of materials on the PET film in order that the PET film may have heat-sealability. In this case, since a bonding agent containing an organic solvent is generally used, the solvent needs to be recovered at a drying step for the purpose of air pollution control. Furthermore, this type of plastic material is desired to be recovered after use to be recycled as a resin material. However, the fact that different types of resin layers are on the PET film prevents the recycle.
In the manufacture of the PET film by a biaxial orientation, an obtained upper limit of a thickness of the PET film is 125 μm at best. Although the thickness of the manufactured PET film could be increased to a range from 350 to 500 μm, the differences in the thickness of the PET film is increased. In view of this problem, non-orientation methods or die-forming methods are carried out when thick plate-shaped products to be manufactured include the PET films with thicknesses of 0.5 mm, 1.0 mm and 2.0 mm respectively. However, plates of PET manufactured by these methods have low heat-resistance (about 100° C.) and extremely low physical strength, whereupon the PET plates cannot be put to practical use. Accordingly, a range of usage of the PET plates will be widened if the PET plates can be manufactured by the biaxial orientation method.